1. Field of the Invention
The present invention pertains to method and apparatus involving thin film ferroelectrics for use in integrated circuits. More particularly, a smooth bottom electrode structure adjacent a ferroelectric thin film layered superlattice material improves the memory retention characteristics of a ferroelectric capacitor through less imprint and longer memory retention time.
2. Statement of the Problem
Thin film ferroelectric materials are used in a variety of nonvolatile random access memory devices. For example, U.S. Pat. No. 5,600,587 issued to Koike teaches a ferroelectric nonvolatile random access memory using memory cells consisting of a ferroelectric capacitor and a switching transistor. U.S. Pat. No. 5,495,438 issued to Omura teaches a ferroelectric memory that is formed of ferroelectric capacitors connected in parallel. The capacitors have ferroelectric materials of different coercive field values and, consequently, can use or store multi-value data. U.S. Pat. No. 5,592,409 issued to Nishimura et al teaches a nonvolatile memory including a ferroelectric layer that is polarized by the impressed voltage between two gates. The polarization or memory storage state is read as a high or low current flow across the ferroelectric layer, which permits nondestructive readout. U.S. Pat. No. 5,539,279 issued to Takeuchi et al teaches a high speed one transistor-one capacitor ferroelectric memory that switches between two modes of operation including a dynamic random access memory ("DRAM") mode and a ferroelectric random access memory ("FERAM") mode.
FIG. 1 depicts an ideal polarization hysteresis curve 100 for ferroelectric thin films. Side 102 of curve 100 is produced by measuring the charge on a ferroelectric capacitor while changing the applied field E from a positive value to a negative value. Side 104 of curve 100 is produced by measuring the charge on the ferroelectric capacitor while changing the applied field E from a negative value to a positive value. The points -E.sub.c and E.sub.c are conventionally referred to as the coercive field that is required to bring polarization P to zero. Similarly, the remanent polarization Pr or -Pr is the polarization in the ferroelectric material at a zero field value. The Pr and -Pr values ideally have the same magnitude, but the values are most often different in practice. Thus, polarization measured as 2Pr is calculated by adding the absolute values of the actual Pr and -Pr values even though these values may differ in magnitude. The spontaneous polarization values Ps and -Ps are measured by extrapolating a linear distal end of the hysteresis loop, e.g., end 106, to intersect the polarization axis. In an ideal ferroelectric, Ps equals Pr, but these values differ in actual ferroelectrics due to linear dielectric and nonlinear ferroelectric behavior. A large, boxy, substantially rectangular central region 108 shows suitability for use as a memory by its wide separation between curves 102 and 104 with respect to both coercive field and polarization.
Presently available ferroelectric materials depart from the ideal hysteresis shown in FIG. 1. Researchers have investigated materials for use in integrated ferroelectric devices since the 1970's, but these investigations have not yet been commercially successful due to the development of materials that depart from the ideal hysteresis. For example, U.S. Pat. No. 3,939,292 issued to Rohrer reports that early studies of ferroelectric materials for use in ferroelectric memories were performed on phase III potassium nitrate. In practice, it turned out that potassium nitrate materials had such low polarizabilities and were so badly afflicted by fatigue and imprint that the materials were practically useless in microelectronic memories. It is nearly impossible to find ferroelectrics that meet commercial requirements. The best materials for integrated ferroelectric devices are switched using a coercive field that can be obtained from conventional integrated circuit operating voltages, i.e., three to five volts ("V"). The materials should have a very high polarization, e.g., one exceeding twelve to fifteen microCoulombs per square centimeter (".mu.C/cm.sup.2 ") determined as 2Pr, to permit the construction of memories having sufficient densities. Polarization fatigue should be very low or nonexistent. Furthermore, the ferroelectric material should not imprint, i.e., the hysteresis curve should not shift to favor a positive or negative coercive field.
FIG. 2 depicts the effects of environmental stress on hysteresis curve 100. Curve 200 shows the effect of fatigue on curve 100. Fatigue reduces the separation between curves 102 and 104 defining central region 108. Central region 108 progressively becomes smaller and smaller with additional fatigue. This change in separation is due to the creation of point charge defects arising in the ferroelectric material as a consequence of polarization switching together with the associated screening effect of the charge defects on the applied field. Thus, fatigue causes the ferroelectric material to wear out over time due to repeated polarization switching.
U.S. Pat. No. 5,519,234 issued to Araujo et al teaches that the fatigue problem of curve 200 is substantially overcome by the use of layered superlattice materials, such as the "layered perovskite-like" materials described in Smolenskii et al "Ferroelectrics and Related Materials," Gordon and Breach (1984). The layered superlattice materials are capable of providing a thin film ferroelectric material wherein the polarization state may be switched up to at least 109 times with less than thirty percent fatigue. This level of fatigue endurance provides a significant advance in the art because it is at least about three orders of magnitude better than the fatigue endurance of other ferroelectrics, e.g., lead zirconium titanate ("PZT") or lead lanthanum zirconium titanate ("PLZT"). Prior layered superlattice material work has been done primarily with the use of a Pt/Ti bottom electrode and layered superlattice material films on the order of 1800 .ANG. thick. The titanium is used as an adhesion layer to prevent peeling of the electrode from the substrate.
According to section 15.3 of the Smolenskii book, the layered perovskite-like materials or layered superlattice materials can be classified under three general types:
(A) compounds having the formula A.sub.m-1 Bi.sub.2 M.sub.m O.sub.3m+3, where A=Bi.sup.3+ Ba.sup.2+, Sr.sup.2+, Ca.sup.2+, Pb.sup.2+, K.sup.+, Na.sup.+ and other ions of comparable size, and M=Ti.sup.4+, Nb.sup.5+, Ta.sup.5+, Mo.sup.6+, W.sup.6+, Fe.sup.3+ and other ions that occupy oxygen octahedra; PA0 (B) compounds having the formula A.sub.m-1 M.sub.m O.sub.3m+1, including compounds such as strontium titanates Sr.sub.2 TiO.sub.4, Sr.sub.3 Ti.sub.2 O.sub.7 and Sr.sub.4 Ti.sub.3 O.sub.10 ; and PA0 (C) compounds having the formula A.sub.m M.sub.m O.sub.3m+2, including compounds such as Sr.sub.2 Nb.sub.2 O.sub.7, La.sub.2 Ti.sub.2 O.sub.7, Sr.sub.5 TiNb.sub.4 O.sub.17, and Sr.sub.6 Ti.sub.2 Nb.sub.4 O.sub.20.
Smolenskii pointed out that the perovskite-like layers may have different thicknesses, depending on the value of m, and that the perovskite AMO.sub.3 is in principal the limiting example of any type of layered perovskite-like structure with m=infinity. Smolenskii also noted that if the layer with minimum thickness (m=1) is denoted by P and the bismuth-oxygen layer is denoted by B, then the type l compounds may be described as . . . BP.sub.m BP.sub.m. . . . Further Smolenskii note m is a fractional number then the lattice contains perovskite-like layers of various thicknesses, and that all the known type l compounds are ferroelectrics.
Despite the tremendous improvements in low fatigue ferroelectrics attributable to layered superlattice materials, there remains an imprint problem that is typified by curve 202 of FIG. 2. Curve 202 shows that environmental stresses can imprint curve 100 by shifting it to the right or left. This imprinting occurs when the ferroelectric material is subjected to repetitive unidirectional voltage pulses. Some imprinting also occurs as a result of normal hysteresis switching. The ferroelectric material retains a residual polarization or bias that shifts sides 102 and 104 in a positive or negative direction with respect to the applied field. Thus, curve 202 has been shifted in a positive direction 204 by repeated negative pulsing of a ferroelectric capacitor. A shift in the opposite direction could also occur due repetitive pulsing by opposite voltage. This type of pulsing represents what happens to the ferroelectric materials as a consequence of repeated unidirectional voltage cycling, such as the sense operations in FERAMs. Imprint can be so severe that the ferroelectric material can no longer retain a polarization state corresponding to a logical 1 or 0 value.
U.S. Pat. No. 5,592,410 issued to Verhaeghe refers to the ferroelectric imprint phenomenon as `compensation.` The '410 patent teaches that the imprint problem can be reversed by pulsing voltage during the write cycle to return the hysteresis loop towards the unimprinted position of curve 100, as compared to curve 202. Thus, the imprint problem is reversed by special write operations in which the pulsed voltage is opposite the switching voltage. Still, the recommended voltage pulsing does not address the entire problem because the imprint phenomenon is a partially irreversible one. The observed imprinting reflects corresponding changes in microstructure of the ferroelectric crystal, e.g., the creation of point charge defects with associated trapping of polarized crystal domains. These changes in microstructure are not all reversible.
FIG. 3 depicts the deleterious effects of fatigue and imprinting on ferroelectric memory read/write control operations. Memory control logic circuits require a minimum polarization separation window, which is represented by shaded region 300. Region 300 must be large enough to produce a sufficient read-out charge for memory operations, e.g., for the operation of memory sense amplifier circuits. An initial 2 Pr separation window 302 declines over the lifetime of the ferroelectric memory device along tracks 304 and 306 until, after about ten years or so of constant normal use, the separation between tracks 304 and 306 is too small for conducting memory operations. This lifetime of normal use follows stress time line 308. Curve 310 is a polarization hysteresis curve from the same material that produced curve 100, but is measured on decline at a point in time along tracks 304 and 306. The remanent polarization values Rms and Rmn correspond to +Pr and -Pr for the fatigued and imprinted material. Rms and Rmn are defined as remanent polarization at zero field in the fatigued hysteresis curve 310. Arrow 312 shows a quantity of positive polarization retention loss, which is primarily due to fatigue. Arrow 314 shows a quantity of negative polarization imprint loss, which is primarily caused by imprint shifting of curve 312 relative to curve 100. Arrow 316 shows a quantity of voltage center shifting of curve 312 relative to curve 100. This voltage center shifting indicates imprinting of the ferroelectric material.
There remains a need for ferroelectric thin film capacitors that resist fatigue well, have long memory retention times, and are substantially free of the imprint problem.